Analog-to-Digital Converters (ADC) are basic building blocks used in several electronic systems to convert analog signals into the digital domain, thus enabling further digital processing on the converted signals. Typical applications include sensors, such as temperature sensors, humidity sensors, pressure sensors, microphones, baseband of radio receivers and digital instrumentation tools.
The energy or the power available for the ADC is limited in several applications, for example in battery-powered systems. Thus, ADC architectures with the highest energy efficiency are preferred. Successive-Approximation ADCs (SAR ADC) exhibit the best energy efficiency but their resolution is typically below 12 bits, thus making them not suitable for applications requiring higher resolution. If higher resolutions are required, Sigma Delta ADCs are preferred. Even if Sigma Delta ADCs show lower energy efficiency than SAR ADCs, Sigma Delta ADCs can offer higher resolution, typically up to 20 bits or more. Circuit designers choose either SAR ADC or Sigma Delta ADC for a particular application based on the suitability parameters.
An ADC is defined by its bandwidth that is, the range of frequencies it can measure, and its signal to noise ratio that is, how accurately it can measure a signal relative to the noise it introduces. The actual bandwidth of an ADC is characterized primarily by its sampling rate, and to a lesser extent by how it handles errors such as aliasing. The dynamic range of an ADC is influenced by many factors, including the resolution (the number of output levels it can quantize a signal to), linearity and accuracy (how well the quantization levels match the true analog signal) and jitter (small timing errors that introduce additional noise).
One of the main performance parameter of an ADC is its resolution, usually expressed in bits. For an ADC with a resolution of n bits and with input range between Vmin and Vmax, i.e., an ADC whose allowed input signal Vin must be Vmin≦Vin≦Vmax, the standard deviation of the noise introduced in the analog-to-digital conversion can be represented by the equation:
            n      std        =          α      ⁢                                    V            max                    -                      V            min                                                2            n                    -          1                      ,      α    =          1              12            
Thus, a lower noise can be achieved by increasing the ADC resolution (n) or by decreasing the ADC input range (Vmin-Vmax).